Tomographic energy dispersive X-ray diffraction apparatus comprising an array of detectors of associated collimators

ABSTRACT

A tomographic energy dispersive diffraction imaging apparatus comprises a radiation source for directing incident radiation ( 1, 3 ) at a sample ( 4 ) mounted on a support, and detection means ( 9, 10 ) mounted for detecting radiation transmitted through the sample ( 4 ) at a given angle to the direction of incidence of the radiation. The detection means comprises an array of energy dispersive detectors ( 9 ) and an array of collimators ( 10 ), such that each energy dispersive detector ( 9 ) has a respective collimator ( 10 ) associated therewith. Each collimator of the collimator array may comprise a plurality of collimator plates with apertures formed therein which are spaced apart along a direction of the transmitted radiation.

The present invention relates to a tomographic energy dispersivediffraction imaging system (TEDDI).

TEDDI is a relatively recently developed tomographic imaging system.Whereas most traditional tomographic imaging systems rely on absorptiveor spectroscopic responses of a material object to invading radiation,TEDDI is unique in providing diffraction data in combination with eitherabsorption or spectroscopic data. The user has the option of whichparameter to display. For instance, pathological soft tissue specimenswould be expected to show a small contrast in absorption whereasdiffraction patterns will be significantly different between healthy anddiseased tissue. The diffraction pattern across a friction stir weld forexample will show the alloying composition and the absorption contrastwill show macroscopic physical defects. As a further example, a ceramicmaterial with a non-uniform spacial doping could be expected to show avariation in the fluorescence of the dopant across the sample but theabsorption contrast map will yield little useful information.

TEDDI is thus a powerful tomographic imaging system and its developmenthas continued over recent years, see for instance Hall et al,“Synchrotron Energy-Dispersive X-ray Diffraction Tomography”, NuclearInstruments and Methods in Physics Research Section B-Beam Interactionswith Materials and Atoms, 140 (1-2): 253-257 April 1998, Barnes et al,“Time and Space-resolved Dynamic Studies on Ceramic and cementationMaterials”, Journal of Synchrotron Radiation 7:167-177 part 3, May 2000,and Hall et al, “Non-destructive Tomographic Energy-dispersiveDiffraction Imaging of the Interior of Bulk Concrete”, Cement andConcrete research 30(3):491-495 March 2000.

In a typical conventional TEDDI system a white beam from a synchrotronor laboratory X-ray source collimated to the desired spacial resolution(typically with a cross section of approximately 50 μm²) is directed ata sample. An energy resolving detector (typically a cryogenically cooledgermanium solid state detector) with associated collimator is positionedto detect X-rays diffracted at an angle appropriate to the sample underinvestigation and the desired structural resolution. The track of theincident X-ray beam through the sample and the angle subtended by thedetector collimator aperture defines the diffracting sample volume,referred to as the diffracting “lozenge”. In order to obtain a 3-D imagethe sample is scanned in the x, y and z directions, (typically in 50 μmsteps) and the energy dispersive diffraction pattern is recorded at eachpoint. Since each diffracting lozenge is well defined in space a 3-Dstructural map can be built up over the whole sample.

A disadvantage of TEDDI systems developed to date is that the process ofassembling a 3-D image (or even a 2-D image) is an extremely slowprocess commonly taking 14 to 16 hours even using synchrotron radiation.This makes existing TEDDI systems impractical for a laboratory basedanalytical tool and unsuitable for medical in-vivo applications.

It is an object of the present invention to obviate or mitigate thedisadvantages of existing TEDDI systems.

According to a first aspect of the present invention there is provided atomographic energy dispersive diffraction imaging apparatus comprising:

a support for a sample;

a radiation source for directing incident radiation at a sample mountedon the support;

detection means mounted for detecting radiation transmitted through thesample at a given angle to the direction of incidence of the radiation;

the detection means comprising:

an array of energy dispersive detectors and an array of collimators,such that each energy dispersive detector has a respective collimatorassociated therewith.

Using a detector/collimator array in accordance with the presentinvention can significantly reduce the time required to obtain an imageof a sample by providing information on a plurality of sample lozengessimultaneously.

According to a second aspect of the present invention there is provideda collimator for collimating incident radiation into a collimated beam,the collimator comprising at least two spaced collimator plates or foilseach provided with a collimator aperture, the collimator apertures ofadjacent collimator plates or foils being aligned in the direction ofthe collimated beam such that incident radiation passing successivelythrough aligned apertures of adjacent collimator plates or foils isthereby collimated.

The collimator structure according to the present invention can readilyprovide a densely packed array of adjacent collimators, achieving highangular resolution, ideal for use in the improved TEDDI system.

The invention also provides a method of constructing the inventivecollimator wherein the collimator apertures are formed by laserdrilling.

Other preferred and advantageous features of the various aspects of thepresent invention will be apparent from the following description.

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a known TEDDI system;

FIGS. 2 a-2 c schematically illustrate embodiments of part of a TEDDIsystem in accordance with the present invention;

FIG. 3 is a schematic illustration of a known collimator structure;

FIG. 4 is a schematic illustration of a collimator in accordance withthe present invention;

FIG. 5 is a schematic illustration of a collimator array in accordancewith the present invention;

FIG. 6 is a schematic illustration of a modification of the collimatorarray of FIG. 5 to avoid cross-talk between adjacent collimators;

FIG. 7 is a schematic illustration of a further collimator array inaccordance with the present invention; and

FIGS. 8 a to 8 e illustrate one method of constructing a collimator inaccordance with the present invention.

FIG. 1 is a schematic illustration of a known TEDDI system. A whiteX-ray beam 1, which may for example be produced by a synchrotron or anX-ray tube is collimated by collimator 2 to produce a beam 3 of thedesired spacial resolution. In a typical system the beam will have across section of 50 μm. The collimated beam 3 is directed at a sample 4which is mounted on a support (not shown) capable of scanning the samplein 3 orthogonal directions (x, y and z directions) in appropriatelysmall steps, typically of the order of 50 μm. A deflected beamcollimator 5 and energy dispersive detector 6 is positioned at an angle2θ to the incident beam 3 (the angle is selected a appropriate to thesample under consideration and the desired structural resolution byapplication of Bragg's Law in a known way). The diffracting samplevolume is the lozenge 7 defined by the track of the incident beam 3 andthe diffracted beam 8 accepted by the collimator 5. The size of thelozenge determines the spacial resolution.

To collect energy dispersive diffraction patterns the detector 6 musthave an energy resolution of the order of 2% or better. Thus, in knownTEDDI systems cryogenically cooled solid-state germanium detectors areused. Conventional cryogenically cooled solid-state germanium detectorsare bulky items, typically around 0.5 metres in diameter, and are veryexpensive costing of the order of £15,000 each.

The present inventors have recognised that a silicon pixel detector chiprecently developed at the CCLRC Rutherford Appleton laboratory inOxfordshire UK provides sufficient energy resolution in a small andrelatively cheap package. The detector chip is described in the paper“Two Approaches To Hybrid X-ray Pixel Array Read Out”, P. Seller,et.al., SPIE Vol. 3774, Detectors for Crystallography and DiffractionStudies as Synchrotron Sources, Jul. 1999 and consists of a 16×16 arrayof 300 μm² pixels each effectively constituting a discrete detectorhaving an energy resolution of the order of 250 eV at 5.9 keV. This iscomparable with the best quality cooled germanium detectors which have aresolution between 120 eV and 180 eV at 5.9 keV. In addition, thesilicon detector count rates are of the order of 1 MHz which is adequatefor tomographic studies.

Furthermore, the present inventors have recognised that by using a pixelarray, detector data from a corresponding array of neighbouring detectorlozenges on one plane through the sample can be obtained simultaneously.This is illustrated schematically with reference to FIGS. 2 a to 2 c.Referring first to FIG. 1, this illustrates the conventional singledetector 6 and collimator 5 measuring data from a single diffractionlozenge 7 at a time. FIG. 2 a illustrates that by arranging a plurality,in this case four, detectors 9 vertically, together with theirrespective collimators 10, data may be collected from four diffractionlozenges 7 simultaneously. In this case the lozenges are neighbouringalong the x direction (equivalent to a corresponding x direction scan ina conventional system).

Similarly, FIG. 2 b illustrates that by arranging an array of detectors9 and collimators 10 horizontally (in the Y direction), and providingthe incident beam as a vertically thin fan shape, data from neighbouringdiffraction lozenges 7 in the Y direction may be collectedsimultaneously (equivalent to a corresponding y-direction scan in aconventional system). In the illustration there are seven detectors (ordetecting pixels) and respective collimators arranged side by sideproviding data on seven neighbouring diffraction lozenges.

FIG. 2 c illustrates the effect of providing a two dimensional array ofdetectors 9 and collimators 10 (again with a thin fan shaped incidentbeam) which provides simultaneous data from a two dimensional array ofdiffraction lozenges, in this case lying in the x-y plane.

It will thus readily be appreciated that by providing an array ofadjacent detectors/collimators, and in particular a two dimensionalarray of detectors/collimators, the number of scanning movements of thesample is reduced. Moreover, if a two-dimensional array of detectors isof sufficient size to “cover” the entire sample then scanning will onlybe required in a single direction (the Z direction in the case of thearrangement shown in FIG. 2 c) to obtain a complete set of diffractionpatterns for all voxels (i.e. lozenges) of the sample. Using the siliconpixel detector chip mentioned above a single chip provides a 16×16detector array and two dimensional detector arrays of almost anypractical size can be provided by positioning a plurality of thedetector chips side by side.

Whilst provision of the required planar, or fan shaped, incident beam isstraightforward, collimation of the diffracted energy beams on thisscale is however problematical. Tolerances on the angular collimation ofthe diffracted beam are small. If the collimator aperture is too largethen the energy resolution of the system will be dominated by theangular aperture of the collimator rather than the intrinsic detectorresolution. For example, in order to preserve an energy resolutioncommensurate with a sample structure spatial resolution of 1 to 1.5Angstroms it is necessary to provide each detector with a collimatorhaving a transmitted divergence of the order of 0.3 mrad or 0.02⁰. Thisis referred to as the angular resolution of the collimator. The requiredcollimator resolution for any particular measurement can be determineddirectly from application of Bragg's Law in a known way with referenceto the required energy resolution, detection angle, and spatialresolution within the sample.

FIG. 3 schematically illustrates the angle of resolution of a simplecollimator structure comprising a bore 11 through an otherwise solidblock of absorbent material 12. The scale is exaggerated for clarity.The angular resolution α is the angle between the most divergent rays13/14which may be transmitted directly through the collimator bore 11.From application of simple geometry it can be seen that the angularresolution α is related to the length (L) and diameter (d) of thecollimator bore by the expression:tan(α/2)=(d/2)/(L/2) which gives:α=2 tan⁻¹(d/L)  (1)

Applying the above expression (1) it can be seen that to achieve anangular resolution of 0.02⁰ the collimator bore must have an aspectratio of approximately 6000:1. Thus for a typical collimated beamdiameter of 50 μM the bore would need a length of approximately 300 mmto give this aspect ratio. However, even the most advanced femto laserdrilling systems currently available are only able to achieve an aspectration of 10:1 on this scale with the required degree of accuracy.

Accordingly, a further aspect of the present invention is the provisionof a new form of collimator which may be applied in many differentapplications but is particularly useful for application in the improvedTEDDI system of the present invention. A simple collimator in accordancewith the present invention is illustrated in FIG. 4. Again the scale isexaggerated for clarity. Rather than provision of a continuous borethrough an otherwise solid block of material as illustrated in FIG. 3,an equivalent aspect ratio is achieved by providing apertures 15 and 16of diameter d in respective thin plates or foils 17 and 18 of absorbentmaterial which are spaced by the required distance L (measured betweenthe front face of front collimator foil 17 and the back face of backcollimator foil 18).

Each collimator foil 17, 18 has a respective angular resolution βdetermined by the aperture diameter d and the thickness t of the foil.Thus, applying expression (1):β=2 tan⁻¹(d/t)

However, the two collimator foils combine to give an overall collimatorresolution of α, which is:α=2 tan⁻¹(d/L)

Thus, if d and L are the same as the dimensions d and L of the simplecollimator structure of FIG. 3, the collimator resolution will be thesame. Accordingly, a collimator resolution of 0.02° can readily beachieved by providing 50 μm diameter apertures in respective collimatorfoils spaced apart so that L=300 mm.

The collimator apertures could be formed by any suitable process, but inaccordance with the present invention are preferably formed by laserdrilling. As mentioned above, modern laser drilling can achieve anaspect ratio of 10:1 which equates, for example, to drilling a 50 μmhole in a 0.5 mm thick foil. A practical limit of current laser drillingtechnology would be of the order of 10 μm holes in a 100 μm thick plateand thus to achieve smaller collimator aperture sizes othertechnologies, such as lithographic techniques, would be required

The collimator foils could be of any suitable material and thickness.For use in a TEDDI system the foils are preferably tungsten, which ishighly absorbent to high energy X-rays, and of a self supportingthickness, e.g. of the order of 0.5 mm.

It will be appreciated that for the improved TEDDI system in accordancewith the present invention an array of closely adjacent collimators isrequired, each collimating diffracted light for a respective one of theclosely adjacent detectors (i.e. detecting pixels of the detector chipidentified above). FIG. 5 schematically illustrates part of a collimatorarray constructed in accordance with the principals explained above inrelation to FIG. 4. In FIG. 5 two adjacent collimators are shown (againnot to scale for clarity) each comprising first and second collimatorapertures 15 a/16 a and 15 b/16 b formed in front and back collimatorfoils 19 and 20. From this figure it can be seen that whilst eachcollimator has an angular resolution of α, there is considerable“cross-talk” between the closely adjacent collimators each of which hasa much larger angular divergence β (this is indicated by the dotted raylines), This cross-talk must be eliminated if the detected measurementsare to be meaningful. This is achieved in accordance with the presentinvention by introducing additional collimator foils intermediate thefront foil 19 and back foil 20.

For instance FIG. 6 schematically illustrates the maximum foilseparation FS between two adjacent collimator foils 21 and 22 which canbe tolerated without cross-talk between adjacent collimators comprisingcollimator apertures 15 a/16 a and 15 b/16 b respectively. Applyingsimple geometry it can be seen that the maximum separation FS is relatedto the angular resolution β of the front foil as follows:tan(β/2)=s/FS which gives:FS=s/tan(β/2)  (2)

-   -   or substituting (d/2)/(t/2) for tan(β/2) gives:        FS=(s t)/d  (3)

It will also be appreciated that the overall collimator resolution ofeach of the collimators formed by apertures 15 a/16 a and 15 b/16 b canbe determined from the expression (1) given above.

For example, assuming that each collimator foil has a thickness t of 0.5mm, that each collimator aperture has a diameter d of 50 μm, and thatthe collimator aperture spacing s is 50 μm (i.e. collimator aperturescentres are spaced by 100 μm) the maximum collimator foil separationavoiding cross-talk FS will be 0.5 mm.

With a foil separation FS of 0.5 mm the effective collimator length Lwill be 1.5 mm. Accordingly, applying expression (1) the angularresolution of each collimator in the array will be 0.38°. It may well bethat this resolution is sufficient for some applications, but for thepresent application in which the required angular resolution wouldtypically be of the order of 0.02° further collimator foils must beadded until the overall collimator length L is sufficient to give therequired aspect ratio L:d (in this case of the order of 6000:1).

For example adding a third collimator foil 23 to the arrangement of FIG.6 gives the arrangement illustrated in FIG. 7 from which it will beappreciated that the maximum separation FS of the second and thirdcollimator foils is determined by the angular diversion of the beamleaving the apertures 15 b and 16 b in the second collimator foil 22,i.e. the angular resolution of 0.38° provided by the combination ofcollimator foils 21 and 22. Thus, assuming the separation of foils 21and 22 is the maximum of 0.5 mm, the maximum separation of foils 22 and23 is FS=15 mm (applying expression (2) above).

Once again, the overall angular resolution of the three collimator foilsis related to the distance between the front face of the frontcollimator foil 21 and the back face of the third collimator foil 23.

Additional collimator foils can be added as necessary until the overallcollimator length is at least equal to the required distance L. Thenumber of collimator foils required is minimised by allowing the maximumseparation FS between adjacent collimator foils without cross-talk.Clearly, more than the minimum number of collimator foils may be used ifdesired.

As mentioned above, laser drilling is the preferred method forfabrication of the collimator structure of the present invention. Asimple construction method is illustrated schematically in FIG. 8 a to 8e. A first collimator foil 24 is mounted on an optical bench 25 (whichmay be of a conventional structure) and an aperture of required diameterd is drilled using a laser 26. Using a femtosecond laser the collimatoraperture may have an aspect ratio 10:1 so that for instance a 50 μmdiameter aperture could be drilled in a 0.5 mm thick foil or a 10 μmdiameter aperture could be drilled in a 100 μm thick foil.

A second collimator foil is then placed on the optical bench between thelaser and the first collimator foil as illustrated in FIG. 8 b. Acollimator aperture is then drilled in the second foil. In this way, thelaser both operates to drill the required apertures and ensure thatapertures of successive collimator foils are accurately aligned.

The process may then be repeated adding additional collimator foils asindicated in FIGS. 8 c to 8 e until the required overall collimatoraspect ratio is achieved.

As a variation to the above method, each collimator foil could beremoved from the optical bench after being drilled allowing the nextcollimator foil to be positioned at the same location as the precedingcollimator foil for the drilling operation so that it is located at thelaser focus. Once all collimator foils have been drilled they can thenbe re-mounted on the optical bench and accurately positioned with therequired separation.

It will be appreciated that the laser can be used to drill an array ofneighbouring collimator apertures in each of the collimator foils, ineither a linear or two-dimensional array, so that the overall collimatorstructure comprises a similar array of individual collimators. In thiscase, the maximum spacing of adjacent collimator plates is calculated asmentioned above to ensure there is no cross-talk between adjacentcollimators.

It will thus be appreciated that a collimator suitable for use with thedetector chip technology mentioned above can readily be constructed forapplication in the improved TEDDI system of the present invention. Itwill, however, be appreciated that collimators according to the presentinventions may have applications in other measurement systems.

1. A tomographic energy dispersive diffraction apparatus comprising: aradiation source arranged to direct a planar or fan shaped beam ofradiation having a thin rectangular cross section, the beam of radiationbeing received at a direction of incidence at a sample; and a detectorconfigured to detect radiation transmitted through the sample at arespective angle to the direction of incidence of the radiation, thedetector comprising, a two-dimensional array of energy dispersivedetectors and a two-dimensional array of collimators, the array ofcollimators being located between the sample and the array of energydispersive detectors, the array of collimators being aligned withrespect to the array of energy dispersive detectors, such that eachcollimator in the array of collimators is aligned with a correspondingone of the energy dispersive detectors, wherein each collimator of thearray of collimators comprises a plurality of aligned collimatorapertures formed in respective collimator plates or foils spaced apartalong a direction of the transmitted radiation.
 2. The apparatusaccording to claim 1, wherein the array of energy dispersive detectorscomprises one or more semiconductor detector chips each comprising anarray of individual detector pixels, each of which comprises a singledetector.
 3. The apparatus according to claim 1, wherein a plurality ofcollimator apertures are provided in each collimator plate or foil todefine an array of individual collimators.
 4. The apparatus according toclaim 3, wherein adjacent collimator plates or foils are spaced so as toavoid cross-talk between adjacent collimators of said array ofcollimators.
 5. The apparatus according to claim 1, wherein said angleis between 0 and 180°.
 6. The apparatus according to claim 1, whereinthe radiation source is provided with an incident radiation collimatorfor collimating incident radiation into a fan shaped beam.